Metal clamp frames and edge hardware remove heat from the sheet perimeter. The margin may remain stiff while the center reaches forming temperature, producing pull-in, whitening, poor corner detail, or a thick transition near the trim flange.
Surface temperature should be mapped from the center to the clamped edge. A repeatable cold band that follows the frame indicates conductive loss rather than a failed heater zone. The effect can change after startup as the frame warms, creating different first-piece and steady-state behavior.
Perimeter heater balance, frame preheating, thermal isolation, and clamp design may be adjusted, but the sheet must still retain adequate edge strength. Heating the frame excessively can soften the grip zone and cause slip. The correct condition is a controlled thermal gradient, not necessarily a uniform temperature to the extreme edge.